Indented letters? (embossed)
The text object must be converted to mesh (ALTc -> "mesh from ..."), or you cannot use boolean on it. If you don't convert it, you cannot neither use the boolean on the other mesh which has to be embossed, because you cannot select the text as target object. When you convert the text to mesh, both can be done.

My suggestion, also, is to operate as logic suggests: select the "other mesh", apply the boolean modifier, as "difference", and select the (converted to mesh) text as target "object".